A rail vehicle is known from EP 0 844 157 B1. The signal line loop is necessary here in order to enable a rail vehicle, which is not equipped with a compressed air brake system and is designed, in terms of its origin, as a tram, to operate on long distance rail routes according to the rules of EBO and UIC.
A compressed air brake system is defined by the fact that braking is triggered by a drop in pressure in an air line which leads through a train. This ensures that braking is always carried out when a defect occurs.
So that a corresponding principle which fulfills the requirements of EBO/UIC can also be implemented, for example, in tram trains, the signal line loop is provided in the trains. The signal line loop is a double line from the end of the train to the start of the train and back again and is therefore composed of a signal generating section and a signal distributing section. Control contacts are connected in series in the signal generating section.
The brakes and the electrical actuation devices, which may be, for example, brake control devices or emergency brake valves, are connected to the signal distributing section. The signal line loop operates according to the quiescent current principle, the brakes being activated if the flow of current in the signal line loop is interrupted. For example, a brake release key is additionally used to short circuit the signal distributing section to the reference potential of the supply voltage of the signal line loop.
Current is usually fed in at the end of the train and signal generating and signal distributing sections are usually connected at the front end of the train where the occupied driver's cab is located.
In contrast, the compressed air brake system does not have the usual dependencies. The feeding point (compressed air supply), control (generation of the drop in pressure) and triggering (for example brake valves) can be connected to the air line in any desired sequence and independently of the start of the train and the end of the train.
When a breakdown occurs, the defective rail vehicle has to be towed away. When there is a compressed air brake system, which is customary in longer-distance rail trains, all that is necessary is to connect the air lines of the defective rail vehicle (broken down vehicle) and the towing rail vehicle (recovery vehicle) to one another in order to ensure that the brakes function throughout. However, if a signal line loop is present, problems occur with the braking.
Specifically, as a result of damage to the electrical circuit which has not yet been detected, the braking capability during the towing operation could be disrupted or even prevented. For example, a connection of the signal line loop to the supply potential, which is caused by a defect, could cause the quiescent current principle to be disabled. For this reason, in the past the signal line loops of the towing rail vehicle and of the towed, defective rail vehicle have generally not been connected to one another. Since it was not possible to brake the towed vehicle, the towing operation was possible only if the operational environment and the applicable regulations permit an exception so that rapid clearing of a section of line which is necessary for operational reasons was possible.
Even if the signal line loop is operationally capable in the broken down vehicle, the continuous signal line loop cannot be formed and the brakes in the broken down vehicle cannot be activated if the recovery vehicle is to push the broken down vehicle. This is due to the customary circuit topology in which the signal line loop in the driver's cab of the broken down vehicle has a connection between the signal generating section and the signal distributing section and as a result the signal line loop cannot be extended forwards without additional switching devices, and thus not without special modes of operation.